The oil and gas drilling industry has been undergoing dramatic technology improvements in the last decade, particularly in MWD (Measurement-While-Drilling), directional and horizontal drilling, improved drilling tools and equipment and improved analysis and monitoring capabilities. The combined effect is that drilling cost has been steadily declining, and directional drilling, particularly high-angle, extended reach, and horizontal drilling have become much more popular, and will further see expanded application in the future.
At the same time, due to cost cutting efforts and down-sizing, more and more wells are being drilled on a "turn-key" basis, whereby service companies are asked to contract the entire drilling project at a predetermined benchmark fee, with huge incentives for faster and better drilling, and similar penalties for incurring drilling problems and drilling delays.
The advent of these turn-key projects forecasts an economic condition under which those service companies that are able to improve the drilling operation will reap major profits. Those companies that do not improve may suffer major losses. One single severe incident of a stuck-pipe can mean a loss of hundreds of thousands of dollars in revenue loss, if not more.
A key in preventing pipe-sticking is to improve the monitoring of the well bore resistance, called the torque and drag. Torque and drag result from contact between the drillstring and the well bore of the directional well.
The current method of torque and drag monitoring is to measure the surface loads only, namely, the hook load and surface torque. Many rigs still rely on crude surface measurements. Some have more advanced axial load and torque measurements.
Additionally, numerical simulations using so-called "torque-drag model" programs are also being employed for checks and as planning tools. These torque-drag simulation models are referred to as "soft-string" models. That is to say, the drillstring is treated as without any bending stiffness. The present inventor introduced the "stiff-string model". This model compares the results of the drag generated by actual BHA (bottomhole assembly) deformation using a BHA analysis program. Significant differences were found between the results of the "soft-string" model and the "stiff-string" model. These differences become more pronounced as the stiffness of the BHA increases, as the clearance decreases, and as the well path becomes more crooked. All these models require very specific and detailed information about the well path and the friction "coefficients", which are very hard to actually determine precisely.
Various U.S. patents have issued to the present inventor in the field of the present invention. U.S. Pat. No. 4,848,144 (issued on Jul. 18, 1989), U.S. Pat. No. 4,972,702 (issued on Nov. 27, 1990), and U.S. Pat. No. 5,044,198 (issued on Sep. 3, 1991) have addressed methods of predicting the torque and drag in directional wells. These patents describe a method for generating an improved torque-drag model for at least the collar portion of the drillstring in a directional oil or gas well. The technique of these patents determines the stiffness of incremental portions of the drillstring, and uses this information, along with the borehole clearance and the borehole trajectory, to determine the contact locations between the drillstring and the sidewalls of the well. The contact force at these determined locations can be calculated, taking into consideration all significant kinematic, external, and internal forces acting on that incremental portion of the drillstring. More accurate torque-drag analysis, provided by the model of these patents, assists in well planning, prediction and control, and assists in avoiding drilling problems. This method serves to reduce total costs for the well.
It is an object of the present invention to provide a method for the monitoring and computing of torque and drag in the well bore.
It is another object of the present invention to provide a method that more precisely determines well bore resistance.
It is another object of the present invention to provide a method of determining well bore resistance with less detailed information about the well bore and the friction coefficients.
It is another object of the present invention to provide a method that allows for the determination of contact locations and the magnitudes of the restraining forces and/or torques.
It is a further object of the present invention to provide a method that allows for the locating of the critical sticking point between the drillstring and the well bore.
It is still a further object of the present invention to provide a method that improves the modelling of the well bore system.
These and other objects and advantages of the present invention will become apparent from a reading of the attached specification and appended claims.